The present invention relates to magnetic recording, and more particularly, the electronic circuitry for decoding data stored on a magnetic medium such as a magnetic disc.
Typical magnetic disc recording systems such as the IBM Model 3330 or the Memorex Model 3670 include a computer driven controller, a disc drive system, a plurality of magnetic recording discs, called a disc pack, and at least one magnetic disc surface having recorded thereon servo or timing signals.
Except during a "read" operation, the timing signals recorded on the servo disc surface are sensed, and a variable frequency oscillator (VFO) is phase-locked through appropriate means such as a phase lock oscillator circuit to the servo signals for supplying clock signals to the controller for write and space count operations. For more details of the operation of such a servo disc system, reference is made to U.S. Pat. No. 3,534,344 by George Santana.
The variable frequency oscillator is locked to the servo data by means of a phase-locked loop/oscillator (PLO). However, during a read operation, the VFO is disconnected from the PLO, and the variable frequency oscillator is locked directly to the raw data which is picked up by a magnetic recording head transducer from a selected magnetic recording disc surface.
By raw data, it is meant that the signals comprise both actual data information signals and, depending upon the particular type of recording code used, timing information signals as well. The nature, frequency, and number of data and timing signals depend upon the type of coding used in the system.
But, regardless of the type of code used in the system, it is the function of the VFO and related circuitry during a read operation to decode the incoming raw digital data by separating the data pulses from the clock pulses. Additionally, the separated data pulses can be relocated in time to remove the effects of bit shift and jitter before being passed to the controller.
For additional information relating to the design and operation of a data-recovery system using a VFO, reference is made to U.S. Pat. No. 3,810,234 to Michael R. Monett.
Though minimized to a high degree, there always exists unavoidable misalignment between the servo disc transducer head and the transducer head used when reading. Consequently, when the VFO is disconnected from the PLO and the raw data from the disc pack is provided to the VFO during a read operation, there will invariably be discrepancies between the phase of the PLO output and the raw data signals. A certain amount of time is required to lock the VFO to the raw data. The amount of time required is determined by the gain of the phase locked loop. During normal operation of the VFO, the loop gain is increased when going from the non-synchronized state to the synchronized state. Specifically, the VFO is synchronized to synchronizing pulses which precede each field of data which form a block of information called a record. This record format will be discussed in greater detail subsequently.
During the transition of locking the VFO to the raw data signals, no data can be accurately read. That is, until the VFO is locked to the raw data signals, it is impossible to decode the data and clock signals. Therefore, it is imperative that this transition time be minimized and VFO be rapidly locked to the raw data signals.
A standard format exists for storing information in magnetic disc file systems. A magnetic disc surface is divided up into "tracks" which comprise a plurality of circular concentric recording bands. A track-accessing mechanism provides minimum time movement of a magnetic transducer head, which functions both in read and write modes, from an existing track at one specified disc radius to another track at a different specified track radius, called for by the disc file controller.
Along each track, information is recorded in the form of blocks of information called records. Within each record are three separate fields which contain data information. These are called the Count, Key, and Data fields. The Count field contains orientation information and the length attributes of the Key and Data fields. The Key and Data fields contain the informational data.
Each field of information recorded on the storage medium, whether a Count, Key, or Data field, has appended to it a 56-bit Error Correction Code (ECC). This code is constructed by performing a mathematical algorithm with the data contained in the field. When a field is retrieved (read), the data in the field and the ECC bits are used as input to another mathematical algorithm and the output of this is examined by the controller to determine if an error has been introduced in the retrieval of the field. At this time, one of three unique situations may exist:
a. No errors have been detected, the information read is correct.
b. An error has been detected but sufficient information can be obtained by mathematical analysis of the residual of the algorithm to determine where (in the field) the error is and what the error is. This is called a correctable error.
c. An error has been detected and sufficient information cannot be obtained by mathematical analysis of the residual of the algorithm to determine where the error exists and/or what the error is.
If situation c) exists, additional attempts are made to read the data. This is accomplished by re-reading the field when it subsequently rotates past the transducer head again. This is called a retry. After a predetermined number of unsuccessful retries, further retries are attempted with the transducer head at positions slightly offset from its normal position in an attempt to read data which may have been recorded slightly off-center. If during any retry, the data error becomes correctable, i.e. situation b) above, then the error is handled as a correctable error and the system exits from the error recovery procedure. If during any retry, the data is read properly, i.e. situation a), the system exits from the error recovery procedure. If the error does not become correctable after all retries of the routine, then it is a non-recoverable error.
Even with the ECC and retry procedure with head offset, some data is not recoverable. One instance where this occurs is where the data has been written by a marginal device, with frequency instability in the write circuitry. Such write devices produce recorded data evidencing the so-called "accordian" effect where the frequency of the data is highly variable. At normal VFO loop gain, there is insufficient time response to recover this data.